The refrigeration cycle as utilized in typical heating, ventilation, and air conditioning (“HVAC”) systems is well-known. Although the specific components comprising an HVAC system may vary depending upon system design architecture and performance specifications, at its essence, the HVAC system is made up of four critical components. In a refrigeration system, a liquefied refrigerant is metered by a thermal expansion or pressure reduction valve into a lower pressure environment of an evaporator. In the evaporator, the refrigerant changes phase from a liquid to a vapor as it absorbs heat from a liquid to be cooled. A compressor then draws the refrigerant vapor from the evaporator, raises its pressure, and discharges the refrigerant into a condenser. In the condenser, the heat absorbed in the evaporator is discarded to a heat sink, and the refrigerant changes phase from a vapor to a liquid. Thereafter the liquefied refrigerant may begin another cycle.
In such HVAC systems, especially in modern, large capacity commercial or industrial systems, it is relatively typical that a gravity-flooded evaporator is utilized. Such evaporators have the advantage of providing relatively large cooling capacities, and are used to cool large commercial or industrial structures such as office buildings, stores, malls, warehouses, factories, and the like.
In essence, an evaporator is a shell-and-tube type heat exchanger. That is to say, an evaporator of this type typically has a plurality of tubes contained within a shell. The arrangement of the tubes is optimized to provide multiple, often parallel flow paths for one of two fluids between which it is desired to exchange heat.
In a flooded evaporator, the tubes are immersed in a second fluid. Heat is transferred between the fluids through the walls of the tubes. For example, in some large capacity HVAC (air conditioning) applications, a fluid, such as chilled water, glycol, or brine, flows through the tubes, and a refrigerant is contained in the volume confined between the heat exchanger shell and the outside surfaces of the tubes. In the case of an evaporator for such an application, the refrigerant cools the fluid by heat transfer from the fluid to the walls of the tubes and, subsequently, to the refrigerant. Transferred heat vaporizes the refrigerant in contact with the exterior surfaces of the tubes.
In a gravity flooded evaporator of the type described, liquid refrigerant is introduced into a lower part of the evaporator shell, and the level of liquid refrigerant in the evaporator shell is maintained sufficiently high so as to assure that each individual tube is immersed below the level of liquid refrigerant in the majority of operating conditions. As the heat is transferred from the fluid flowing inside the tubes to the refrigerant, the refrigerant is caused to boil, with the vapor passing to the surface, where it is subsequently withdrawn from the evaporator by suction of the compressor.
In the more typical commercial or industrial HVAC system under consideration herein; however, a refrigerant fills the evaporator tubes, and air is directed over and across the tubes by large fans. In this regard, forced air essentially “immerses” the tubes and allows the above-described heat transfer to occur.
In order to maintain an adequate feed supply of liquid refrigerant to the evaporator tubes, and in order to provide a receptacle for collection of withdrawn, vaporized refrigerant, such systems often require the introduction and integration of a conventional, separately field-piped, surge vessel. Sometimes also known as a surge drum, a dual-state pressure vessel, an accumulator, or the like, a surge vessel, which is plumbed into the low pressure side of an HVAC system, and at an elevation greater than the evaporator, essentially provides the dual function of a separation chamber and an overflow container into which liquid refrigerant may be collected for recirculation to the evaporator; and, into which refrigerant that is in gaseous state may be collected simultaneously for return to the compressor.
Consistent with this functionality, a surge vessel may also act to absorb surges in refrigerant from the evaporator, such as may occur as a result of operational load variance. For example, lighter loads produce less vapor; thereby, allowing a greater liquid component within the tubes. On the other hand, heavier loads produce more vapor; thereby, reducing the liquid component within the tubes. Thus, a surge vessel provides a receptacle into which excess liquids or vapors may be driven according to the operational load characteristics of the evaporator.
Finally, a surge vessel may act to capture the sometimes violent liquid surges that may occur when the suction line of a flooded evaporator is opened. When the line is opened, a pressure drop occurs. This pressure drop flashes a quantity of liquid refrigerant into vapor, and this vapor can nearly instantly force the entire remaining charge of liquid into the surge vessel.
Thus, a surge vessel operates to buffer liquid and vapor refrigerant surges within the HVAC system; to collect dual-state (to wit; liquid and vapor) refrigerant from the evaporator; to separate that refrigerant according to its state; and to, thereafter, allow the refrigerant, if in liquid state, to be recirculated by gravity directly into the evaporator, or, if in gaseous state, to be recompressed and condensed into liquid state for subsequent return to the evaporator.
As will become readily apparent, a system configured in accordance with the description above, or which is similar thereto by virtue of the presence of a gravity flooded evaporator and a conventional, separately field-piped, surge vessel, is disadvantageous for a variety of reasons. For example, in prior art systems using surge vessel technology, there is a significant user-borne cost due to the requirement for a separately field-piped, expensive, pressure rated and pressure tested, certified, surge vessel and associated subsystem. There is a potential for decreased system quality and reliability, due to field-piping and installation errors in connecting the surge vessel into the system. In that regard, it takes time and resources to design, procure, install, and inspect such a field-piped subsystem, taking into consideration requirements for compliance with boiler and pressure vessel codes, additional valving and piping requirements, initial and life-cycle pressure testing, ongoing maintenance, cyclical component replacement, and the like, all of which are associated with installation and operation of an HVAC system with a surge vessel.
Furthermore, there are greater installation and maintenance costs due to increased required volumes of liquid refrigerant to charge the evaporator and surge vessel, with associated regulatory concerns and associated procurement and disposal costs. Overall system footprint and cost is increased due to the additional space requirements for the surge vessel and associated piping.
Still further, in a system with a surge vessel, there is a differential in the static height of the liquid refrigerant under varying operational load states, which may tend to reduce evaporator capacity due to added net saturation pressure and/or temperature. There are energy losses by virtue of the compressor needing to compress a larger volume of vapor, which results in greater work and higher operational costs due to increased energy consumption.
Thus, considering the above-described disadvantages inherent within an industrial HVAC system comprising a gravity flooded evaporator and a conventional, separately field-piped, surge vessel, it would be preferable to provide, and to be able to recognize the benefits of, a system not requiring use of a separately field-piped surge vessel. It is to the provision of such an HVAC system that the present disclosure is now directed.